This invention relates generally to a transformer, and more particularly to a variable inductance transformer for high voltage applications.
Variable inductance transformers are well known devices that provide adjustment to the inductance across the transformer secondary winding. Variable inductance transformers are particularly useful for high voltage applications such as, for example, spark testers which apply high voltage to insulated conductors to detect defects in the insulation such as pinholes through the insulator or other types of defects in the insulator. The amount of inductance across the secondary winding to maintain a predetermined output voltage is partly a function of the capacitance and thickness of the insulator. Conventional variable inductance transformers typically use a variable air gap in the core to adjust the inductance across the secondary winding of the transformer to maintain the output voltage. The size of the air gap is varied, for example, by placing shims in the gap or otherwise mechanically varying the gap. A drawback with these transformers is that high voltage across the secondary of the transformer may drop below a desired level as the capacitance of the object under inspection varies. Further, the current generated at the secondary may rise to levels which could electrocute an operator coming into accidental contact with the output terminal of the transformer.
In view of the foregoing, it is a general object of the present invention to provide a variable inductance transformer which overcomes the above-mentioned drawbacks and disadvantages associated with prior variable inductance transformers.
In one aspect of the present invention a variable inductance transformer includes a core defining a gap between opposing first and second ends. A primary winding and at least one secondary winding are coupled to the core. The secondary winding is provided for stepping up the voltage across the primary winding. A carriage assembly includes a magnetic shunt movable against the core and variably across the gap. The magnetic shunt has a width at least as wide as the gap for moving the shunt to a predetermined position along the core in a range from an uncovered position not overhanging the gap, through intermediate positions overhanging the gap, to a covered position where the shunt bridges the gap to adjustably vary the inductance of the secondary winding to resonate with a load capacitance.
In another aspect of the present invention a variable inductance transformer system includes a variable inductance transformer having a core defining a gap between opposing first and second ends. The transformer includes a primary voltage input winding, a secondary voltage output winding, and a tertiary voltage test winding. The secondary winding is for generating an output signal having a stepped up voltage relative to that received by the primary winding, and the tertiary winding is for generating a signal having a reduced and proportional voltage relative to that generated by the secondary winding. A carriage assembly includes a magnetic shunt movable against the core and variably across the gap. The magnetic shunt has a width at least as wide as the gap for moving the shunt to a predetermined position along the core in a range from an uncovered position not overhanging the gap, through intermediate positions overhanging the gap, to a covered position where the shunt bridges the gap to adjustably vary the inductance of the secondary winding to resonate with a load capacitance. Means are provided for moving the carriage assembly to position the magnetic shunt against the core.
Preferably the moving means includes a motor having a drive shaft threadably engaging the carriage assembly. The drive shaft is rotatable in clockwise and counterclockwise directions to bidirectionally move the magnetic shunt carried by the assembly to a desired position along the core. Further, a contact surface of the magnetic shunt opposing the core preferably terminates at an edge extending along an oblique angle from a first end to a second end relative to a direction along a longitudinal length of the gap, whereby the gap is progressively bridged by the magnetic shunt from its first end to its second end when the magnetic shunt is moved over the gap. The progressive bridging of the gap is to prevent sudden changes to the inductance of the secondary winding.
The moving means also preferably includes a phase detector having inputs communicating with the input and output sides of the transformer, as well as a servo amplifier having an input coupled to an output of the phase detector, and an output coupled to a control input of the tuning motor for controllably energizing the tuning motor to move the magnetic shunt into a position along the core so that the voltage signals at the input and output sides of the transformer are in a predetermined phase relation to each other. In the preferred embodiment the voltage signals at the input and output sides of the transformer are in phase with each other, and a resistive element, such as an incandescent lamp is placed in series with the primary winding of the transformer to limit the current across the secondary winding should a short circuit occur at the secondary.
An advantage of the present invention is that the inductance of the transformer is automatically varied to bring to or maintain the output voltage at a high value.
Another advantage of the present invention is that the output voltage is maintained at a predetermined level despite fluctuations in the input voltage.
A yet further advantage is that the current at the secondary is limited in the case of short circuit to prevent electrocution.
These and other advantages of the present invention will become more apparent in the light of the following detailed description and accompanying figures.